Transposable elements (TEs) are acknowledged sources of genetic change within organisms. The effects of transposition can range from the disruption or creation of a single gene to large-scale genome rearrangements. Transposition events can result in beneficial mutations which allow an organism to adapt to a new environment. In the last three years, several studies have reported that some miRNAs, small RNAs involved in post-transcriptional gene regulation, have evolved from TEs. miRNAs play an important role in the stress responses of many organisms. Interestingly, TEs are derepressed under the same stress conditions that miRNAs are known to ameliorate. The observation that miRNAs are known to evolve from TEs and that TEs are derepressed under stress conditions lead me to question whether TEs play a role in environmental adaptation through the creation of small RNA networks. To test this idea, Chlamydomonas reinhardtii cultures were grown under low carbon, nitrate enriched conditions and the small RNA pool was analyzed. I found that these conditions do stimulate the expression of novel small RNAs and that some of these RNAs and their targets are derived from transposition events.

Tout organisme vivant est le produit d'interactions complexes entre son génome et son environnement, interactions caractérisées par des échanges de matière et d'énergie indispensables à la survie de l'organisme et la transmission de son génome. Depuis la découverte dans les années 1910 que le chromosome est le support de l'information génétique, les biologistes étudient les génomes afin de décrypter les mécanismes et processus à l'oeuvre dans le développement des organismes et l'évolution des populations. Grâce aux améliorations technologiques des dernières décennies, plusieurs génomes ont été entièrement séquencés, leur nombre s'accroissant rapidement, mais ils sont loin d'être décryptés pour autant. En effet, certains de leurs composants, les éléments transposables, sont encore mal compris, bien qu'ils aient été détectés chez quasiment toutes les espèces étudiées, et qu'ils puissent représenter jusqu'à 90% du contenu total de leurs génomes. Les éléments transposables sont des fragments du génome possédant la particularité d'être mobiles. Ils ont donc un impact majeur sur la structure des génomes mais également sur l'expression des gènes avoisinants, notamment via des mécanismes épigénétiques. Leur évolution est aussi particulière étant donné qu'ils ont une transmission verticale non-mendélienne et que de nombreux cas de transferts horizontaux ont été mis en évidence. Mais, à part dans le cas de certains organismes modèles pour lesquels nous disposons de séquences de référence, l'annotation des éléments transposables représente souvent un goulot d'étranglement dans l'analyse des séquences génomiques. A cela s'ajoute le fait que les études de génomique comparée montrent que les génomes sont bien plus dynamiques qu'on ne le croyait, en particulier ceux des plantes, ce qui complique d'autant l'annotation précise des éléments transposables. Pendant mes travaux de thèse, j'ai commencé par comparer les programmes informatiques existants utilisés dans les approches d'annotation de novo des éléments transposables. Pour cela, j'ai mis au point un protocole de test sur les génomes de Drosophila melanogaster et Arabidopsis thaliana. Ceci m'a permis de proposer une approche de novo combinant plusieurs outils, capable ainsi de reconstruire automatiquement un grand nombre de séquences de référence. De plus, j'ai pu montrer que notre approche mettait en évidence les variations structurales au sein de familles bien connues, notamment en distinguant des variants structuraux appartenant à une même famille d'éléments transposables, reflétant ainsi la diversification de ces familles au cours de leur évolution. Cette approche a été implémentée dans une suite d'outils (REPET) rendant possible l'analyse des éléments transposables de nombreux génomes de plantes, insectes, champignons et autres. Ces travaux ont abouti à une feuille de route décrivant de manière pratique comment annoter le contenu en éléments transposables de tout génome nouvellement séquencé. Par conséquent, de nombreuses questions concernant l'impact de ces éléments sur l'évolution de la structure des génomes peuvent maintenant être abordées chez différents génomes plus ou moins proches. Je propose également plusieurs pistes de recherche, notamment la simulation des données nécessaires à l'amélioration des algorithmes de détection, démarche complémentaire de la modélisation de la dynamique des éléments transposables.

Transposable elements (TEs) are found in nearly all eukaryotic genomes and are a major driving force of genome evolution. The hAT superfamily of TEs are found in a variety of organisms, including plants, fungi, insects and animals. To date, only 14 hAT TEs in the Aedes aegypti genome have been annotated as having a hAT transposase coding sequence. In this study, extensive bioinformatic approaches have been employed to find hAT TEs that encode transposases in the A. aegypti genome. A total of six newly-identified TEs belonging to the hAT superfamily were discovered in the A. aegypti genome. Furthermore, a computer program called TE Displayer was developed to analyze TEs in genome sequences. TE Displayer detects TE-derived polymorphisms in genome datasets and presents the results on a virtual gel image. TE Displayer enables researchers to compare TE profiles in silico and provides a reference profile for experimental analyses.

Transposable elements (TEs) are found in nearly all eukaryotic genomes and are a major driving force of genome evolution. The hAT superfamily of TEs are found in a variety of organisms, including plants, fungi, insects and animals. To date, only 14 hAT TEs in the Aedes aegypti genome have been annotated as having a hAT transposase coding sequence. In this study, extensive bioinformatic approaches have been employed to find hAT TEs that encode transposases in the A. aegypti genome. A total of six newly-identified TEs belonging to the hAT superfamily were discovered in the A. aegypti genome. Furthermore, a computer program called TE Displayer was developed to analyze TEs in genome sequences. TE Displayer detects TE-derived polymorphisms in genome datasets and presents the results on a virtual gel image. TE Displayer enables researchers to compare TE profiles in silico and provides a reference profile for experimental analyses.

Transposable elements (TEs) are one major force of genome evolution thanks to theirability to create genetic variation. TEs are ubiquitous and their proportion is variable between species and also populations, suggesting that a tight relationship exists between genomes and TEs. The model system composed of the natural populations of the twin sisters Drosophila melanogaster and D. simulans is interesting to compare host/TE relationship, since both species harbour different amounts of TE copies. The helena element is nearly silenced in D.simulans natural populations despite a very high copy number. Such repression is associated to abundant internally deleted copies suggesting a regulatory mechanism of TEs based on DNA deletion. Another pathway of TE regulation is through epigenetics where the host genome is able to keep intact the DNA sequences of TEs and still silence their activities.Chromatin remodelling is well known in drosophila and specific histone modifications can be associated to specific chromatin domains. We observed an important variation on H3K27me3and H3K9me2, two heterochromatic marks, on TE copies in D. melanogaster and D. simulans natural populations. Also, we show that derepressed lines of D. simulans exist for specific elements, have high TE transcription rates and are highly associated to non constitutive heterochromatic marks. TEs are therefore controlled by the host genome through DNA deletion and a possible chromatin remodelling mechanism. Not only genetic variability is enhanced by TEs but also epigenetic variability, allowing the host genome to be partitioned into chromatin domains. TEs are therefore mandatory to gene network regulation through their ability of "jumping epigenetics".

Thesis (Ph.D.)--University of Alabama at Birmingham, 2009. / Title from PDF title page (viewed on Feb. 2, 2010). Includes bibliographical references (p. 64-72).

Diss. (sammanfattning) Stockholm : Univ., 2007. / Härtill 4 uppsatser.

Transposable elements (TEs) are mutagenic mobile DNA sequences whose excision and insertion are powerful drivers of evolution. Some TE families are known to target specific genome features, and studying their insertion preferences can provide information about both TE biology and the state of the genome at these locations. To investigate this, collecting large numbers of insertion sites for TEs in natural populations is required. Genome resequencing data can potentially provide a rich source of such insertion sites. The field of detecting these "non-reference" TE insertions is an active area of research, with many methods being released and no comprehensive review performed. To drive forward knowledge of TE biology and the field of non-reference TE detection, we created McClintock, an integrated pipeline of six TE detection methods. McClintock lowers the barriers against use of these methods by automating the creation of the diverse range of input files required whilst also setting up all methods to run simultaneously and standardising the output. To test McClintock and its component methods, it was run on both simulated and real Saccharomyces cerevisiae data. Tests on simulated data reveal the general properties of component methods' predictions as well as the limitations of simulated data for testing software systems. Overlap between results from the McClintock component methods show many insertions detected by only one method, highlighting the need to run multiple TE detection methods to fully understand a resequenced sample. Utilising the well characterised properties of S. cerevisiae TE insertion preferences, real yeast population resequencing data can act as a biological validation for the predictions of McClintock. All component methods recreated previously known biological properties of S. cerevisiae TE insertions in natural population data. To demonstrate the versatility of McClintock, we applied the system to Drosophila melanogaster resequencing data. 27 Schneider's cell lines were sequenced and analysed with McClintock. In addition to demonstrating the scalability of McClintock to larger genomes with more TE families, this exposed ongoing transposition in S2 cell lines. Likewise, the use of non-reference TE insertions as variable sites allowed us to recreate the relationships between S2 sub-lines, confirming that S1, S2, and S3 were most likely established separately. The results also suggest that there are several S2 sub-lines in use and that these sub-lines can differ from each other in TE content by hundreds of non-reference TE copies. Overall this thesis demonstrates that the McClintock pipeline can highlight problems in TE detection from genome data as well as revealing that much can still be learned from this data source.

Since their first discovery in the 19th century, microsporidian species have been found to be successful obligate intracellular parasites capable of infecting a wide variety of hosts including economically and ecologically important organisms as well as model organisms for biomedical research. Recently, cases of infection of the widely used animal model Danio rerio, commonly known as the zebrafish, by the newly described microsporidium Pseudoloma neurophilia have been reported in an increasing number of research facilities. Current knowledge of the biology of this parasite found in 75% of the Zebrafish Resource Center facilities is limited to microscopic analyses on its lifecycle as well as its physical, behavioral and psychic impact on its hosts. Despite the growing relevance of this parasite in biomedical research no current data is available on its genome. In this dissertation, I provide additional knowledge on the basic biology of P. neurophilia by acquiring and exploring the content and structure of the first genome draft of the zebrafish parasite. My findings reveal that the 5.25 Mb genome of P. neurophilia harbors an unusually high amount of transposable elements as well as numerous inserts found in coding regions typically conserved in microsporidia and other organisms. This peculiar obligate parasite demonstrates strong phylogenetic and genetic relationships with other fish-mosquito microsporidia. Similar to what is observed in closely related species, intra-genomic analyses of P. neurophilia’s genome suggest that it is diploid and possesses a large repertoire of over a thousand putative genes unique to this specie. Overall, my findings provide new insights into the basic biology of this parasite and represent a milestone in the understanding of P. neurophilia and D. rerio host-parasite interaction and ultimately in the development of treatments against this parasite that has been infecting the zebrafish research industry for the past decades.

Transposable elements (TEs) are mobile DNA sequences that are a source of mutations and can target specific sites in host genome. Understanding the molecular mechanisms of TE target site preferences is a fundamental challenge in functional and evolutionary genomics. Here we used accurately mapped TE insertions in the Drosophila melanogaster genome, from large-scale gene disruption and resequencing projects, to better understand TE insertion site mechanisms. First we test predictions of the palindromic target site model for DNA transposon insertion using artificially generated P-element insertions. We provide evidence that the P-element targets a 14 bp palindromic motif that can be identified at the primary sequence level that differs significantly from random base composition in the D. melanogaster genome. This sequence also predicts local spacing, hotspots and strand orientation of P-element insertions. Next, we combine artificial P-element insertions with data from genome- wide studies on sequence properties of promoter regions, in an attempt to decode the genomic factors associated with P-element promoter targeting. Our results indicate that the P-element insertions are affected by nucleosome positioning and the presence of chromatin marks made by the Polycomb and trithorax protein groups. We provide the first genome-wide study which shows that core promoter architecture and chromatin structure impact P-element target preferences shedding light on the nuclear processes that influence its pattern of TE insertions across the D. melanogaster genome. In an effort to understand the natural insertion preferences of a wide range of TEs, we then used genome resequencing data to identify insertions sites not present in the reference strain. We found that both Illumina and 454 sequencing platforms showed consistent results in terms of target site duplication (TSD) and target site motif (TSM) discovery. We found that TSMs typically extend the TSD and are palindromic for both DNA and LTR elements with a variable center that depends on the length of the TSD. Additionally, we found that TEs from the same subclass present similar TSDs and TSMs. Finally, by correlating results on P-element insertion sites from natural strains with gene disruption experiments, we show that there is an overlap in target site preferences between artificial and natural insertion events and that P-element targeting of promoter regions of genes is a natural characteristic of this element that is influenced by the same features has the artificially generated insertions. Together, the results presented in this thesis provide important new findings about the target preferences of TEs in one of the best-studied and most important model organisms, and provide a platform for understanding target site preferences of TEs in other species using genomic data.